Forward/Filter Decisions
When a frame arrives at a switch interface, the destination hardware address is compared to
the forward/filter MAC database. If the destination hardware address is known and listed in
the database, the frame is only sent out of the appropriate exit interface. The switch won’t
transmit the frame out any interface except for the destination interface, which preserves
bandwidth on the other network segments. This process is called frame filtering.
But if the destination hardware address isn’t listed in the MAC database, then the frame
will be flooded out all active interfaces except the interface it was received on. If a device
answers the flooded frame, the MAC database is then updated with the device’s location—
its correct interface.
If a host or server sends a broadcast on the LAN, by default, the switch will flood the
frame out all active ports except the source port. Remember, the switch creates smaller
collision domains, but it’s always still one large broadcast domain by default.
In Figure 10.3, Host A sends a data frame to Host D. What do you think the switch will
do when it receives the frame from Host A?